In most real signal transmission applications there can be several sources of noise and distortions between the source of the signal and its receiver. As a result, there is a strong need to correct mistakes in the received signal. As a solution for this task one should use some coding technique with adding some additional information (i.e., additional bits to the source signal) to ensure correcting errors in the output distorted signal and decoding it. One type of coding technique utilizes low-density parity-check (LDPC) codes. LDPC codes are used because of their fast decoding (linearly depending on codeword length) property.
During media flaw scans, which typically occur during the manufacture and assembly of storage devices, drive makers write test patterns over the entire media and use flaw detection circuits in the read channel to identify defects in the media. This information is used to determine the final drive format. One common traditional test pattern is a 4-T-periodic repeating binary pattern.
After manufacture, drive makers require that even uninitialized sectors (sectors that do not yet contain user data) of the media can be read and recovered during drive operation. Because traditional flaw scan test patterns are not true low-density parity-check (LDPC) codewords, they cannot be used for this purpose in an LDPC-based drive system. Instead, drive makers typically write dummy data sectors over the entire media following the flaw scan, requiring a second write of the entire media.
In order to reduce manufacturing time by eliminating the second full-media write, drive makers are exploring the possibility of using run-length limited (RLL) LDPC-based test patterns for media flaw scans. Such test patterns are also decodable in the field. However, a general LDPC-based test pattern may contain runs of Nyquist or DC patterns that interfere with the read channel ability to detect media defects reliably.
Most flaw detection is based on detecting some unexpected change in signal quality; loss of amplitude is a commonly used flaw detection metric. Due to the low-pass nature of the over-all system, Nyquist patterns have very low signal amplitude even when there is no defect on the media. Effectively, a Nyquist pattern does not provide enough nominal signal amplitude in order to effectively detect changes in signal quality. DC patterns can provide adequate signal amplitude, but do not provide any phase information for phase-based detection algorithms.
Consequently, it would be advantageous if an apparatus existed that is suitable for generating LDPC-based test patterns with minimal Nyquist and DC pattern runs.